1. Field of the Invention
The present invention relates to a battery monitor apparatus for monitoring the voltage of each cell of a battery pack configured with a plurality of cells.
2. Description of the Related Art
In the prior art, a secondary battery (a chargeable/dischargeable battery such as a lithium ion battery) is used as a battery pack (including a plurality of cells (battery cells) connected in series and parallel) for supplying power to various portable equipment. When the charge/discharge of the battery is repeated, especially of the lithium ion battery, it is necessary to monitor the voltage of the whole battery pack and the over-charge/over-discharge of each cell and to take preventive measures such as stopping the charge or discharge as required.
For this purpose, in the circuit shown in FIG. 13A, when an over-charge comparator circuit detects that the voltage of the whole battery pack has increased beyond a specified voltage corresponding to the number of cells constituting the battery pack at the time of charge, the charge is stopped. At the time of discharge, on the other hand, the discharge is stopped when an over-discharge comparator circuit detects that the voltage of the whole battery pack has decreased to below a specified voltage corresponding to the number of cells constituting the battery pack.
Also, in the circuit shown in FIG. 13B, an operational amplifier (OP amplifier) is attached to each cell making up the battery pack, so that upon detection of the potential of the positive electrode of each cell, the voltage of each stand-alone cell is calculated, by subtracting the potential of each cell sequentially in the OP amplifier, and notified to a microprocessing unit (MPU) not shown. In the event that the voltage of each stand-alone cell deviates from a specified voltage range during charge or discharge, the discharge or charge, as the case may be, is stopped, a cell voltage failure is detected, an alarm is issued or other appropriate measure is executed for controlling the prevailing state.
In spite of this requirement, the conventional circuit shown in FIG. 13A, though simple in configuration, harbors the problem that it is impossible to detect for each stand-alone cell whether the charge/discharge voltage for each cell has deviated from a specified range or not.
Also, in the conventional circuit shown in FIG. 13B, the voltage of each cell is calculated by an OP amplifier connected to each cell and other OP amplifiers for subtracting the outputs of the OP amplifiers. With the increase in the number of cells, the OP amplifiers also increase in number. Further, it is difficult to regulate the individual offsets accurately in view of the characteristics of OP amplifiers made up of analog elements. This leads to the problem that an increased number of cells increases the voltage error of each cell due to the offset error. Another problem is that the offset regulation circuit, if attached to each OP amplifier, complicates the circuit configuration greatly and requires a large space.
Still another problem is the voltage imbalance at the time of charge or discharge between a plurality of cells in series of the battery pack. Especially during charging, an over-charged cell may exist. Nevertheless, the state of each cell (over-charged or over-discharged or normal) is not detected nor displayed individually, thereby giving rise to the problem of an unknown state.